For digital video compression, motion compensated inter-frame coding is a very effective compression technique and has been widely adopted in various coding standards, such as MPEG-1/2/4 and H.261/H.263/H.264/AVC. In most current coding systems, a macroblock, consisting of 16×16 pixels, is primarily used as a unit for motion estimation and subsequent processing. Nevertheless, in the recent development of the next generation standard named High Efficiency Video Coding (HEVC), a more flexible block structure is being adopted as a unit for processing. The unit of this flexible block structure is termed as coding unit (CU). The coding unit can start with a largest coding unit (LCU) and is adaptively divided into smaller blocks using a quadtree to achieve a better performance. Blocks that are no longer split into smaller coding units are called leaf CUs, and data in the same leaf CU share the same coding information. The quadtree split can be recursively applied to each of the LCUs until it reaches the smallest CU, the sizes of the LCU and the smallest CU (SCU) are properly selected to balance the tradeoff between system complexity and performance.
In the H.264 coding standard, the underlying video frames are divided into slices, where each slice consists of non-overlapping macroblocks as the smallest coding unit. Each slice can be coded as an I-slice (intra-coded slice), P-slice (predictive slice) or B-slice (bi-directional slice) and the compressed data are packed into slice-layer data. Since the slice is independently processed, errors or missing data from one slice cannot propagate to any other slice within the picture. Furthermore, redundant slices are permitted for robust transmission. In the recent HEVC development, the slice contains multiple LCUs instead of macroblocks. The LCU size being considered is 64×64 pixels which is much larger than the macroblock size of 16×16 pixels. Compared with the macroblock aligned slice for H.264, the LCU-aligned slice for HEVC does not provide enough granularities for dividing video frames. Consequently, it is very desirable to develop flexible slice structure where slice partition is based on smaller coding units and to develop mechanism to convey flexible slice structure information between an encoder and a decoder.